This invention relates to a seat post assembly for cycles, including a seat post formed of carbon fiber or other relatively soft high strength-to-weight material, and a protective tubular sleeve formed of a hard metal material and arranged concentrically about the lower end of the seat post, thereby protect the seat post against damage during the vertical adjustment thereof relative to the cycle frame.
The use of lightweight componentry, such as a seat post, in bicycles, motorcycles, and other cycles is often seen as an opportunity to reduce the total weight in order to increase the efficiency of the cycle. However, since cycle components (i.e., a seat post) are relied upon to support the weight of the rider, the strength of the component cannot be compromised in order to reduce the weight of the part. As a result, cycle component manufacturers have used advanced materials with high strength-to-weight ratios, such as carbon fiber, titanium, magnesium, beryllium and high strength aluminum, to produce lightweight component parts. The state of design for light-weight cycles such as bicycles, though, depends upon the compatibility of the componentry to work on a wide variety of frame designs, and within various industry standards in regards to size and specification. That being the case, component manufacturers are limited in their ability to optimize the component design for the use of such advanced materials. In addition, many bicycles are assembled and serviced by untrained mechanics and consumers who might compromise the structural integrity of a component formed from advanced materials through improper installation.
Seat posts are particularly at risk of being compromised by the previously mentioned threats and limitations due to the described function of the seat post. The necessary adjustability, compatibility with various frame designs, and the fact that the seat post supports the greatest percentage of the rider""s weight during travel make the use of advanced materials potentially dangerous should the seat post fail during use.
Over the years manufacturers have produced very lightweight seat posts (200 g or less for a seat post 300 mm or longer) with pillars made from carbon fiber, high strength aluminum alloys and titanium. Historically, in-the-field failures of these kinds of seat posts have been common. In addition to improper installation and abusive riding, a common cause of seat post failure has been the irregular deformation of the clamping devices used to secure seat posts into the bicycle frame. The out-of-round shape that results from the clamping device being secured can create a stress riser on the seat post at the point where it enters the bicycle frame. This is also the point of the peak bending moment during the dynamic loading of the seat post in use. As a result, the ultimate strength of the part can be greatly reduced and brought into the range of typical service loads. This affect is even more compromising in materials that are particularly notch sensitive, such as high strength aluminum and carbon fiber.
In order to counter the notch sensitivity of such materials many seat post manufactures have turned to using internal reinforcements to the seat post pillar either made from the same or different materials as the pillar or integral to the seat post pillar itself. However, if this kind of reinforcement is used over the entire length of the pillar, the weight advantage of using an advanced lightweight material may be lost; and if the reinforcement is used in only part of the pillar, there is the possibility that the rider will adjust the pillar so that a non-reinforced portion of the pillar is subject to the stress riser created by the clamping device.
Titec Cycle USA, Inc. developed a carbon fiber seat post called xe2x80x9cthe C-1 91xe2x80x9d. Originally, it was believed that a carbon fiber composite pillar could be developed that would not require reinforcement. Nonetheless, the conclusion was arrived at that a sufficiently lightweight composite tube could not be developed that had adequate hoop strength to withstand the stress riser created by the seat tube clamping device. As a result, the decision was made to install an extruded aluminum alloy on the interior of the composite pillar in order to reinforce the tube in a limited clamping area defined by a graphic on the outer diameter of the seat post pillar. This reinforcement eliminated most of field failures of the seat post, but not all. Many consumers would insert the seat post into their bicycle frame past the designated clamping area, where the post would fail under a substantial load.
Another manufacturer, Easton Sports, Inc., introduced the xe2x80x9cCT2xe2x80x9d carbon fiber seat post, which was integrally reinforced through the use of a variable wall thickness. This variable wall thickness design also required that a limited clamping area be defined by a graphic on the outer diameter of the seat post. However, multiple field failures drove Easton to perform multiple product redesigns, which extended the reinforced area of the seat post pillar and increased the product""s weight. Additional field failures then lead to the incorporation of two flats on the cross-sectional outer diameter of the seat post. These flats provided a relief from the stress riser created when the seat tube clamping device deforms.
The Ochoa U.S. Pat. No. 5,888,214 discloses the provision of a compression device between a cycle frame and a cycle seat. A metal interacts with a compression rebound unit within a housing which is slidably inserted into the hollow cylindrical bicycle seat post mast and is secured in the mast with a set screw, thereby cushion the ride of the user.
Carbon fiber composites can be optimized by their fiber orientation to withstand substantial loads, and provide strength-to-weight ratios superior to almost any other know materials. However, that optimization is best achieved when the fiber orientation is almost entirely unidirectional. If the seat post application requires multi-directional strength characteristics, then additional layers of the carbon fiber fabric maybe required in the lay-up in order to meet the strength requirements and the weight advantage will be lost. In addition, since there is a range of seat post diameters that are considered standard, anywhere from 25.4 mm to 31.8 mm, it is difficult to optimize the lay-up for each size. It is also cost prohibitive to create tooling and to stock inventory for seat posts in each of the various standard sizes.
Accordingly, a primary object of the present invention is to provide a seat post assembly for cycles, including a seat post having a protective sleeve concentrically secured to the exterior of the lower end of the seat post, thereby to protect the seat post when inserted within the seat tube of a cycle frame. The sleeve further provides a mounting surface for the seat post within the cycle frame to regulate the height adjustment of the saddle height relative to the cycle frame to ensure a safe height adjustment. The sleeve is secured to the outer diameter of the seat post with the length of the sleeve being sufficient to accommodate the normal range of adjustment required to fit the rider to their bicycle, thereby leaving the remainder of the carbon fiber seat post exposed. The insertion of the seat post is limited by the length of the sleeve, since the outer diameter of the carbon fiber seat post is smaller than could be secured by the bicycle frame""s clamping device.
By using the outer diameter of the sleeve to mate to the interior diameter of the seat tube of the frame, the carbon fiber seat post and saddle clamp assembly parts could be common in the production of seat posts in each of the twelve industry standard sizes. Only the outer diameter of the sleeve would have to be changed, and the sleeves could be easily machined from one or two extrusion sizes.
The reduced diameter necessary for the carbon fiber seat post to fit within the common interior diameter of the sleeve makes the seat post more flexible. This added flexibility, in conjunction with natural damping characteristics of the carbon fiber material, reduce the vibration and shock normally transmitted to the rider through the seat post and produce a more comfortable ride. The reduced fatigue on the rider allows him to perform at a higher level of efficiency over a longer period of time.